1. Field of the Invention
The present invention relates in general to a hollow structural member of a body structure of a motor vehicle, and more particularly to such a hollow structural member provided with an internal reinforcement.
2. Discussion of the Related Art
The body structure of a motor vehicle includes various stationary hollow structural members each of which includes a hollow body portion such as tubular body portion. These hollow structural members include impact beams, side door waists, center pillars and front pillars. The mechanical strength of each hollow structural member can be increased by a reinforcement disposed inside the body portion. For instance, the impact beam disposed within a side door of a passenger vehicle is provided to minimize inward deformation of the side door due to an impact load applied thereto laterally of the vehicle body upon collision of the vehicle at the side door with a given object. Generally, the impact beam includes a tubular body portion having a circular transverse cross sectional shape, and a pair of fixing portions integrally fixed to the longitudinal opposite ends of the body portion. The impact beam is fixedly attached at its fixing portions to the frame of the side door, such that the body portion of the impact beam extends in the longitudinal or running direction of the vehicle. There has been proposed an impact beam of the type in which a reinforcement is disposed inside a longitudinal central part of the tubular body portion, in order to increase the bending strength of the central part so as to prevent buckling thereof, while minimizing an increase in the weight of the impact beam. Examples of this type of impact beam are disclosed in JP-A-4-238727 and JP-6-91325, which use a tubular reinforcement or a cylindrical solid reinforcement. The buckling of the impact beam is interpreted to mean bending of the tubular body portion into a curved flattened shape due to an impact load laterally applied thereto. The bending strength is a value of the load at which the buckling or fracture of the impact beam occurs.
In the conventional tubular structural member as described above, the bending strength or rigidity considerably changes at and near the opposite ends of a reinforcement provided in the tubular body portion, so that the stress tends to be concentrated around the ends of the reinforcement, increasing the possibility of cracking or fracture at the corresponding parts of the tubular body portion. Thus, the conventional tubular structural member suffers from a problem of insufficient improvement in the bending strength.
Explained more specifically referring to FIGS. 9 and 10, a test on an impact beam 10' (not provided with a reinforcement) using a pendulum 30 revealed a stress distribution as indicated in the graph of FIG. 10. In the test, the end face of the pendulum 30 was forced onto the impact beam 10', at a longitudinal center point S of the impact beam 10'. A FEM (finite-element method) analysis showed the stress distribution of FIG. 10 in which the stress continuously decreases in the opposite longitudinal directions of the impact beam 10', with an increase in the distance from the point (S) of application of the load, as indicated at (1) through (4) in FIG. 9. The stress has a maximum value amax at the load application point or center point S. In the graph of FIG. 10, the distance (mm) from the load application point S is taken along the abscissa (right and left direction in FIG. 9), while the value of the stress on the upper side of the impact beam 10' is taken along the ordinate. It will be understood from the graph of FIG. 10 that a stress value .sigma.40 at the positions 40 mm away from the load application point S is still considerably large. Accordingly, where a reinforcement having a length of about 80 mm is disposed within the tubular body portion of the impact beam 10' such that the opposite ends of the reinforcement are spaced about 40 mm away from the load application point S, stress concentration takes place at or near the opposite ends of the reinforcement, and the impact beam 10' is likely to be fractured. It is considered that the use of a reinforcement having a sufficiently large length is effective to reduce the stress values around the ends of the reinforcement and prevent the fracture of the impact beam 10'. However, this potential solution inevitably results in an undesirable increase in the weight and material cost of the reinforcement.
JP-A-7-506067 discloses the use of a reinforcement in the form of a generally elongate plate whose width dimension decreases in the longitudinal opposite directions toward its opposite ends, with a decrease in the bending moment applied to the elongate plate. This reinforcement also suffers from considerable changes of the bending strength around the longitudinal ends of the elongate plate. Further, the elongate plate whose width dimension is still comparatively large at the longitudinal ends tends to have cracking due to stress concentration at its end portions. There is also proposed to reinforce a local portion of a tubular structural member such as a center pillar or a side sill of a vehicle body, by using a reinforcement consisting of two metal plates which are welded together and disposed within the tubular body portion of the structural member. This type of reinforcement also suffers from drawbacks as described above.